Gravimetry is a type of measurement that has been used for reservoir characterization and gas-movement monitoring. Some commercially available gravimeters have been based on Newton's theory of gravitation. For example, Lacoste-Romberg gravimeters use a zero-length spring to monitor the gravitational force on a small test mass. Some gravimeters monitor the time of flight of a free-falling object or the oscillation time of a pendulum. More recently, some gravimetry measurements monitor the position of a superconducting niobium sphere suspended by a magnetic field.
In contrast to Newton's theory of gravitation, Einstein's theory of gravitation introduces a general relationship between a geometric structure of space-time and the presence of massive bodies. Einstein's theory predicts a time dilatation in the presence of a massive body. Such time dilatation is referred as “Gravitational Red Shift”.
Nuclear spectroscopy offers a process that has been used to observe and confirm the gravitational red shift predicted by Einstein's theory of gravitation. For example, the isotope 57Fe nucleus can emit from its lowest excited state a 14.4 keV photon. This state can have an approximate lifetime of 140 nanosecond (ns), a spectral-line width of about 10−8 eV or a relative line width of about 9.2×10−13. The Mössbauer effect recognizes that a nuclear transition has a high frequency precision, or narrow spectral line width, such as that of the 57Fe nucleus. The Mössbauer effect has successfully been used to measure/verify the gravitational red shift, as predicted by Einstein's theory of gravitation. Pound and Rebka in 1960 were the first to use a Mössbauer experiment to quantitatively confirm the gravitational red shift of Einstein's theory of gravitation.